1) A music venue/bar are not necessarily the same thing. It comes down to capacity/exposure of the venue/song performed, respectively, so there is no general estimate that would be relevant to both a bar that could hold 50-100 people and also a legit music venue that holds 1,000-3,000 people. Bigger venues/stadiums are again another thing - the price will vary with all three.

2) Going back to #1 , the way these things are supposed to work are by proportionality and expectancy. Based on the demographics of your venue, a PRO will determine what you would likely have to pay if you were to pay for each song individually, then determine where that money most likely would go. It's the same if you are a PRO member as an artist - I might not get paid each instance for a play on my royalty statement, and I might get paid some amount because there was money in "the pot" that was unaccounted for, and at the time it seemed likely some of that could be owed to me.

3) You have to deal with each PRO individually, since each one will have an exclusive contract with an artist or a portion of the artist's catalogue. Anyone not affiliated with a PRO you would have to negotiate with individually, and that is even more unpredictable than dealing with on of the big organizations.

4) PRO's essentially answer to no one, so even if you find a "standard rate", as a venue/bar owner you would be helpless to hold them to that if they decided to deviate from it. They decide what you would likely have to pay and offer that as a blanket fee. There may be levels or categories to their pricing, but each license they grant is essentially unique.

5) If you wanted to try to pay "per song", either for a one-off event, or to try to skimp the system, you would pay significantly more than you would otherwise. That is why they offer blanket fee's in the first place. For a one time event it might make sense to pursue this avenue, though it would also be a much less streamlined, and much more difficult process. For a public venue owner, it is even more of a hassle.

6) PRO's are not necessarily trustworthy, and their main goal IMO is not looking out for the best interest of a bar/venue owner, nor the musicians/songwriters they represent. It is a self-preserving system, and they decide the rules. They have the resources to bankrupt most independent artists/venues if they so choose, so if you play that game you play by their rules.

This is a very complicated - though misguided - question, and you won't get any real answers without speaking to an entertainment attorney who specializes in this area. If you're in a situation where this is actually relevant to you - get a lawyer.